On the flip side, lowering global emissions and limiting future global warming would reduce the Southwest's risk of prolonged droughts, the researchers found in a paper published Wednesday in the journal Science Advances.

"Megadroughts are virtually certain in a business-as-usual greenhouse gas emissions scenario," Toby Ault, the study's lead author and an assistant professor at Cornell University in New York, told Mashable.

"There really still are benefits to adopting an aggressive [emissions] mitigation strategy to reduce the risk of megadroughts," he added.

The paper builds on the team's earlier findings that rising carbon emissions could drastically increase the risk of decades-long droughts, which would further strain the region's already limited water supplies and spark substantially more forest fires, dust storms and tree diseases.

The 2015 NASA-backed study found that, by the second half of this century, the Southwest and Central Plains could see droughts that are drier and longer than those experienced in the region in the last 1,000 years.

"Everything is just warming up, so it's going to get harder to store moisture in the soil, even if it rains more."

Wednesday's paper is the first to put a precise number on that risk. It also helps to clarify which factors will likely spur the future megadroughts.

Ault said the team found that regional temperature increases alone will push megadrought risks to 70 percent, 90 percent or 99 percent by 2100.

Future precipitation patterns, by contrast, will have virtually no effect. The researchers found, to their surprise, that regional rainfall could increase, decrease or stay the same, and it wouldn't change their estimates for megadrought risks.

Maps of megadrought risk for the American Southwest under different levels of warming, and the required increase in precipitation to compensate for that warming. Image: science advances

That's because the rising air temperatures would drive higher evaporation rates on the ground; the warmer the soil, the less likely it is to lock in moisture. Rainfall would have to rise at least 50 percent in the Southwest in the coming decades — which nobody is projecting will happen — to compensate for the rise in regional temperatures.

Their finding that precipitation won't stem megadrought risk is significant given the broader uncertainty about how rainfall patterns will change due to global warming, said Samantha Stevenson, a project scientist at the National Center for Atmospheric Research in Boulder, Colorado, who wasn't involved in the study.

"This model shows that uncertainty in precipitation doesn't really matter, as much as the fact that megadrought risk is going to increase," she told Mashable.

"Everything is just warming up, so it's going to get harder to store moisture in the soil, even if it rains more," she said.

When megadroughts struck the Southwest in the distant past, the impacts on water supplies and agriculture were enough to destroy or break apart prehistoric civilizations. Around the 13th century, a megadrought forced the Ancient Puebloans (also called Anasazi) to abandon their dwellings in the Four Corners region of Arizona, Colorado, New Mexico and Utah.

Modern Americans might not face such a catastrophic conclusion to megadroughts. Southwestern residents could still haul in water from over hundreds of miles away or move to less arid communities without spurring the decline of U.S. society, Ault said.

But the region would still likely suffer substantial consequences from megadroughts, including a more severe version of the drought conditions they're experiencing today, said Jonathan Overpeck, co-director of the University of Arizona's Institute of the Environment in Tuscon.

He noted that the Southwest is enduring a 16-year-long drought, an event that's putting enormous strain on the Colorado River, which provides water supplies for major cities including Phoenix, Las Vegas, Salt Lake City, Denver, Los Angeles and San Diego.

Dry conditions have also killed off broad swaths of trees, or made them more vulnerable to deadly beetle infestations. The dead timber and parched soil have also enabled a wave of dangerous, sweeping forest fires in recent years.

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